Regional Express, the country airline better known as Rex, donated up to $100,000 in free flights to help the major political parties campaign at the last election, raising new questions about how airlines and other regulated companies curry favour in Canberra.

Pressure is also mounting on Coalition transport spokeswoman Bridget McKenzie to reveal how many free flight upgrades she has not declared after The Australian Financial Review revealed she had written to the three major airlines asking for the information, and sources said there were at least two.